Referring to FIGS. 1(a) and (b), which show the output carrier signals OUTA and OUTB of a push-pull type Class-D audio amplifier. The output carrier signals OUTA and OUTB of the push-pull type Class-D audio amplifier have the same carrier phase as shown in FIG. 1(a), or have a constant phase shift therebetween (for example 180°) as shown in FIG. 1(b). In general use, the output carrier signals will be filtered by a filter or human ears, therefore the synchronous design for the output carrier signals is not necessary.
Referring to FIG. 2, which shows schematically a circuit for Class-D audio amplifier. An audio signal Vin is inputted from left to the two input terminals “a”, “b” of an operational amplifier 1. The operational amplifier 1 outputs two audio signals respectively to operational amplifier 2 and 3 (both are comparators), and then pass through the predriver 4 and 5 for driving power CMOS circuits 6 and 7 to generate output signals OUTA and OUTB. Output signals OUTA and OUTB drive the loudspeaker 8 cooperatively, this is a push-pull type circuit, and is what so-called BTL (Bridge Tied Load) design. Resistors 9 and 10 are used for feedbacking signals; capacitors 11 and 12 are compensation circuits; inductors 13, 14 and capacitors 15, 16 are filter circuits. Triangular wave Vtriangular of 500 KHz is inputted to operational amplifiers 2 and 3 for synchronizing the two carrier signals inputted to the operational amplifiers 2 and 3, therefore the phases of the carriers of output signals OUTA and OUTB will be the same with each other, as shown in FIG. 1(a); or the phases of the carriers of output signals OUTA and OUTB will have a phase shift of 180°, as shown in FIG. 1(b). The operational amplifiers 1, 2 and 3 cooperatively form a PWM (Pulse Width Modulator) for the Class-D audio amplifier. A PWM has varieties of design. FIG. 2 shows only one of them.
An additional triangular wave generator is required for generating the triangular wave Vtriangular, but a triangular wave generator is very complicated in design and is not very easy to generate very accurate triangular wave. An additional triangular wave generator will occupy too much space in an IC design, thus increasing cost. The triangular wave Vtriangular for synchronizing the two carrier signals of the output signals OUTA and OUTB is not really necessary.
Moreover, feedback signals of the output signals OUTA and OUTB via resistors 9, 10 to the operational amplifier 1 must be very accurate, otherwise the operational amplifiers 1 cannot keep its two output audio signals in balance, but this requirement causes a narrow tolerance in IC layout, thus increasing manufacturing cost.